connect: Performs a TCP connect to a server. Useful for testing non-supported protocols.
dns: Checks DNS servers for correct resolution of host names.
domain: Checks DNS servers for consistent and correct behavior for a specific domain.
ftp: Checks the FTP protocol that is used in file transfers.
ftps: Checks the FTP protocol that is used in file transfers from SSL-secured FTP servers.
http: Checks the HTTP protocol that is used for non-secured web sites.
https: Checks the HTTPS protocol that is used on SSL-secured web sites.
imap: Checks the IMAP protocol that is used for email retrieval.
ldap: Checks the LDAP protocol that is used to access remote directory services.
ping: Sends PINGs (ICMP 'echo request' packets) and checks packet loss.
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pop3: Checks the POP3 protocol that is used for email retrieval.
scp: Checks the SCP protocol that is used in file transfers over SSH.
sftp: Checks the FTP protocol that is used in file transfers over SSH.
sip: Checks the SIP protocol that is used for VoIP, Instant Messaging, presence applications etc.
smtp: Checks the SMTP protocol that is used for (outgoing) email transmission.
telnet: Checks the TELNET (remote login) protocol.